1. Field of the Invention
The present invention relates to a screw design, particularly to a screw with low drilling resistance for speeding up the drilling performance and enhancing the drilling force.
2. Description of the Related Art
Referring to FIGS. 1 to 3, a “Thread-Forming Screw Fastener” 1 disclosed in the US Publication No. 2005/0186048 comprises a head portion 11, a shank portion 12 extended from the head 11, a plurality of threads 13 spiraling on the shank 12, and a drilling portion 14 disposed on the shank 12, opposite to the head 11. Wherein, a plurality of wave troughs 131 are defined on the threads 13. Especially, a first apex angle α included by an upper flank 132 and a lower flank 133 of the threads 13 is formed between 30°˜35°, and a second apex angle α′ included by wave crest 135 formed on the outer thread edge 134 of the deepest region of the wave troughs 131 is contoured between 30°˜58°. Further, a trough angle A is included by the wave trough 131. Accordingly, the screw 1 utilizes the concurrent drilling and cutting efficiency provided by the wave troughs 131 to reinforce the severing performance of the threads 13, thence achieving a speedy drilling and fastening effect.
However, in time of the threads 13 practically drilling into an object 2, the dimension of the trough angle A of the wave troughs 131 as well as the dimension of the first apex angle α of the threads 13 readily influences fastening effect of the screw 1 though the wave troughs 131 offer a preferable cutting function. The analysis is as follows:    1. The dimension of the trough angle A influences the fastening effect:
The dimension of the trough angle A decides the cutting function of the threads 131. That is, if the trough angle A is formed by an improperly small size, each wave trough 131 could not be guaranteed to provide the auxiliary cutting with efficiency during the threads 13 drilling into the object 2. Because the elasticity is characterized by the fibers contained in the object 2, the fibers are not able to respectively get into the small-sized wave troughs 131. Therefore, the cutting efficiency of the wave troughs 131 is limited or even the drilling of the threads 13 is in vain; thereby the screw 1 is incapable of being well-drilled into the object 2. On the other hand, if the trough angle A is formed by an improperly large size, the dimension of the threads 13 is likely to be relatively diminished. Herein, such drilling and cutting efficiency of the larger trough angle A is not as satisfied as that of the smaller trough angle A. Moreover, the smaller threads 13 having a weaker and thinner formation that adversely results in an insufficient bearing force readily collapse.    2. The first apex angle α of the threads 13 influences the fastening effect:
The first apex angle α decides the fastening effect of the screw 1. That is, if the first apex angle α is formed by an improperly small size, the threads 13 are unable to bear a large drilling torque while they confront a more rigid object, which readily breaks the threads 13. On the other hand, if the first apex angle α is formed by an improperly large size, the threads 13 merely bears a larger drilling torque but provides the cutting effect inferior to that of one with a smaller first apex angle. Thus, the utilization is adversely limited, which indeed needs amendment.